Cardiac rhythm management devices, which include pacemakers and implantable cardioverer/defibrillators (ICD), are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction.
Patients with sinus node dysfunction who are unable to maintain an adequate atrial rate (i.e., are chronotropically incompetent) but have intact intrinsic AV conduction are most appropriately treated with atrial pacing only such as AAI mode. (See the pacing modes section below for an explanation of the three-letter code for defining pacing modes.) Although ventricular pacing could also be delivered to these patients, it is preferable from a hemodynamic standpoint to utilize the native AV conduction system for stimulating the ventricles so that optimum atrio-ventricular synchrony is maintained. Also, several major clinical studies have demonstrated that unnecessary ventricular pacing is associated with a significantly increased risk of heart failure and atrial fibrillation in patients with dual-chamber pacemakers and ICDs.
Many patients with sinus node dysfunction, however, are at a higher risk than normal of developing another conduction system disorder such as some degree AV block. In AV block, the conduction of excitation from the atria to the ventricles is either slowed for blocked completely, resulting in atrioventricular dyssynchrony. Most physicians are therefore reluctant to program a pacemaker to AAI mode in patients with sinus node dysfunction because of the risk that they may at some time develop an AV block and need ventricular pacing. One way around this is to program the pacemaker with a dual-chamber mode which paces the atria and then the ventricles after a programmed AV delay interval (e.g., DDD mode) and specify a very long AV delay interval so that the ventricles are never paced as long as intrinsic AV conduction is intact. Such a mode, however, due to the length of the AV delay interval and its associated sensing refractory periods, may cause unacceptable loss of atrial and ventricular sensing capability which interferes with tachyarrhythmia detection. The programmed long AV delay interval also results in long nonphysiological AV delays causing atrioventricular dyssynchrony when ventricular pacing is required, which is clinically detrimental if it occurs for prolonged periods (referred to a pacemaker syndrome).